Repeaters are a cost-efficient way to increase the coverage in a wireless communications system. Several repeater nodes can be placed within the coverage range of a particular base station. The repeater nodes extend the effective coverage of the base station, that is, the size of the cell, by retransmitting the signal received from the base station.
Repeaters are sometimes used to provide indoor coverage from an outdoor base station. The attenuation in walls is in the order of 10-20 dB, which means that without a repeater the signal strength experienced by indoor terminals may be very small. A simple repeater may be used to amplify the signal and create an indoor signal strength of an appropriate level.
Frequency and time diversity are fundamental parts of a communications system.
The conditions for wireless communication inside a building typically differ significantly from those outdoors.
The term diversity gain refers to the increase in the signal field strength in a cell that can be achieved by means of diversity, for example, frequency or time diversity. A diversity gain can be achieved by means of coding and interleaving in the time and frequency domain. The diversity gain can also be realized by means of scheduling. In this case the channel quality to each user is measured, and a scheduler allocates data only to users having good channel conditions. Assuming that the channel conditions are good for all users at some time each user can receive data when his channel conditions are good. In this way, the waste of system resources, for example, transmission power is minimized.
Frequency selective fading is encountered in scenarios where a large delay spread is present. This is usually the case in large macro cells. In contrast, indoor scenarios often have a nearly flat fading in the frequency domain. Hence, the gain of frequency domain coding and scheduling is limited in an indoor scenario.
In the same way it can be argued that time domain selectivity may be very different between for example, an indoor and an outdoor cell. This is because indoor users and their surroundings are relatively stationary, while outdoor users are more likely to move. Also, surrounding objects such as cars also contribute to time selectivity. The rate of time selectivity is important in system design, since this is related to system parameters such as slot length, pilot density etc.
Also, since the requirements from different areas of a cell may be very different, it may be impossible to optimize system parameters for all users in a cell. In particular the requirements from outdoor and indoor users will typically be very difficult. Similar problems may arise if a cell covers an outdoor cell comprising areas with very different topographies.